Rotary atomizers are one type of apparatus used commercially to apply liquid coating materials in atomized form onto substrates. Apparatus of this type generally include an atomizing cup, a motor for rotating the atomizing cup at high speeds, a source of liquid coating material such as paint which is delivered to the atomizing cup, and, in some applications, a high voltage power source for applying an electrostatic charge to the atomized paint particles. Liquid coating material is delivered to the interior of the atomizing cup and flows along its inner wall under the application of centrifugal force. When the coating material reaches the peripheral edge or atomizing lip of the cup, it is flung radially outwardly to form atomized particles of coating material. In recent years, the trend has been to increase the speed of rotation of the atomizing cup to speeds on the order of 10,000 rpm to 40,000 rpm, or higher, in order to effectively atomize liquid coatings which are normally difficult to atomize, and to increase the quantity of coating material which can be atomized by a single rotary atomizer.
One problem which has been encountered with rotary atomizers of the type described above is that foam or bubbles in the atomized coating particles can be created, particularly at high speeds of operation. The presence of foam or bubbles in the atomized particles causes defects in the coating applied to a substrate, such as a roughened appearance and/or a haze that destroys the gloss on the substrate surface. It is theorized that such defects result from the production of entrapped air in at least some of the atomized coating particles which causes these particles to foam.
This problem has been addressed in high speed rotary atomizers of the type disclosed in U.S. Pat. Nos. 4,148,932 and 4,458,844. These patents are directed to rotary atomizers having an atomizing bell or cup formed with a plurality of grooves or notches near the peripheral edge of the cup which extend in a radial direction and increase in depth in the direction of the flow of coating material along the inside surface of the cup. These grooves divide the flow of coating material into separate streams, as opposed to an essentially continuous sheet of coating material on the inside surface of the cup. It has been found that such individual streams are more readily atomized without the formation of entrapped air in the atomized particles, and thus produce a more acceptable coating on a target substrate.
One problem with apparatus such as disclosed in U.S. Pat. Nos. 4,148,932 and 4,458,844 is that radial grooves reduce the structural integrity of the peripheral edge of the atomizing bell or cup. As a result, the cup can be relatively easily damaged during use. Another problem with such apparatus is that complete separation of the coating material into individual streams may not be obtained, particularly at relatively high flow rates of the coating material. The construction of the atomizing bell or cup as disclosed in U.S. Pat. Nos. 4,148,932 and 4,458,844 results in the formation of areas of the inside surface of the cup, between adjacent radial grooves, which are in the same plane as the flow of coating material along the cup surface. While much of the coating material flows into the grooves for separation into streams, some of the coating material might nevertheless continue to flow along the areas of the inside of the cup between grooves and thus interfere with the formation of separated, individual streams of coating material for atomization.
A third potential problem with rotary atomizers of the type described in U.S. Pat. Nos. 4,148,932 and 4,458,844 above is pressure loss. As the coating material moves along the inside surface of the cup toward its peripheral edge, centrifugal force pressurizes the coating material. The sudden pressure drop which occurs when the coating material is flung from the atomizing lip of the cup atomizes the coating material, and the effectiveness of such atomization is at least partially dependent upon maintaining the coating material at high pressure up to the atomizing edge or lip. By forming grooves in the atomizing bell or cup upstream from the atomizing lip of the cup, a pressure loss occurs before the coating material is discharged from the atomizing lip which can adversely effect atomization.